The present invention relates to a method for biologically removing phosphorus and nitrogen and an apparatus therefor, more particularly, to a method for biologically removing phosphorus and nitrogen using granulated methan-oxidizing bacteria and an apparatus therefor.
In general, an apparatus for biologically removing phosphorus and nitrogen using suspended microbes includes an anaerobic tank, an anoxic tank, an aeration tank, and a settling tank. In the anaerobic tank, phosphorus inside cells is released, and nitrate-nitrogen or nitrite-nitrogen is reduced to nitrogen gas and removed in the anoxic tank. The aeration tank removes organic materials, oxidizes nitrogen-based components such as nitrate-nitrogen, and excessively takes phosphorus released from the anaerobic tank. The settling tank settles the suspended microbes so as to separate treated water and the microbes from each other. This type of apparatus removes phosphorus by repeating sequential anaeration and aeration processes in which sludge in the anaerobic tank undergoes the anoxic tank, flows into the aeration tank, and returning to the anaerobic tank. Also, processes of removing oxygen from sludge and exposing the sludge to air need to be performed repeatedly to remove nitrogen.
High concentration of organic materials expressed by biological oxygen demand (BOD) or chemical oxygen demand (COD) in inflow water is necessary for easy proceeding in releasing phosphorus and removing nitrate-nitrogen from a target material in anaerobic and anoxic tanks, which are reaction tanks for removing phosphorus and nitrogen. If inflow water has low or insufficient concentration of organic materials, an additional reaction tank needs to be put after the aeration process to remove nitrogen, and methanol or acetic acid needs to be supplied as a supplementary source of organic carbon. If methanol or acetic acid is not decomposed during the denitrification, an additional aeration process is performed to completely decompose methanol or acetic acid. Therefore, if inflow water has an insufficient amount of carbon, additional costs are usually required to install additional denitrification and aeration tanks and purchase expensive pharmaceutical products such as methanol or acetic acid. Accordingly, another source of carbon that can replace methanol or acetic acid needs to be developed.
Methane gas is produced from organic waste landfills and anaerobic sludge hydration tanks in sewage treatment plants. In particular, methane gas is often considered as a global warming gas, and usually burnt in air. Thus, methane may be a cost-effective substitutional source of carbon for denitrification.
When methane is supplied into an aeration tank, which is an aerobic reaction tank, along with the air to use the methane as a source of carbon, methan-oxidizing bacteria start living and instigate sequential reactions as follows. First, methanotrophs converts the methane into an organic material such as methanol with use of oxygen dissolved in water. Second, methylotrophs reduces nitrate-nitrogen in water into nitrogen gas using the methanol as a source of carbon.
The conventional method is contrived to improve a nitrogen removal rate based on characteristics of methan-oxidizing bacteria, which usually grow in an aerobic condition. More specifically, in the activated sludge process using suspended microbes, inflow waste water, air, and methane gas are directly input into an aeration tank, and in a biological medium that uses adhering microbes, inflow waste water, air, and methane gas are directly input in a reaction tank. Therefore, in the conventional removal method of nitrogen and phosphorus based on methan-oxidizing bacteria, methan-oxidizing bacteria often grow along with bacteria used to remove organic materials expressed in COD and BOD, nitrogen-oxidizing bacteria used to oxidize ammoniac nitrogen. As a result, methan-oxidizing bacteria may not grow dominantly.
Since bacteria that oxidize organic materials grow faster than methan-oxidizing bacteria, reaction products such as methanol produced by methan-oxidizing bacteria (i.e., methanotrophs) are consumed first by bacteria that oxidize organic materials. Thus, a nitrogen removal rate of methylotrophs is low. Also, methan-oxidizing bacteria produced by methane gas generally produce more gelatinous materials such as extra polysaccharide (EPS) than the conventional suspended or adhering microbes by 10-fold or more. Thus, when biological media such as pebbles and ceramics are used, openings between the biological media are frequently closed. Methane gas is one of global warming gases and is explosive when methane gas of more than 5% exists in air. Hence, when the conventional method is employed, the concentration of discharge gas is high after a biological reaction; thus, the discharge gas may become explosive.
Accordingly, it is necessary to develop a method and an apparatus that allows dominant growth of methan-oxidizing bacteria, prevents closure of openings when a biological medium is used, and obtain a low concentration of methane in a discharge gas after a biological reaction in order to effectively use methane gas as a source of carbon for removing nitrogen.